The present invention relates to an optical amplifier used in a multiplexed optical transmission path, a method of controlling a multiplexed light output delivered out of the optical amplifier, an optical transmission system using the optical amplifier and a method of monitoring and controlling the optical transmission path for transmission of the multiplexed light output.
In recent years, with the demand for reducing the cost of an optical communication system, a so-called wavelength multiplexing optical transmission scheme has been studied in which two or more kinds of signal light beams of different wavelengths are multiplexed and transmitted through a single optical transmission fiber. Since the optical amplifier has a wide amplifying wavelength band and can afford to perform amplification at low noise, it is suitable for use as an amplifier in the wavelength multiplexing optical transmission. A rare earth added optical fiber constituting the optical amplifier and a semiconductor optical amplifier have each the gain which has wavelength dependency and therefore, there occurs a difference in light output or gain between wavelengths after amplification. Specifically, the inter-wavelength difference is accumulated in the course of multi-stage relay based on optical amplifiers and after the relay, the difference in optical power between wavelengths is extended. As a result, the maximum relay transmission distance of the whole system is limited by a degraded S/N ratio of a wavelength of the multiplexed wavelengths which has the lowest power. Accordingly, it is of importance to provide an optical amplifier which does not cause the light output difference between wavelengths.
Thus, for example, a scheme described as xe2x80x9cFlattening of Multi-wavelength Amplifying Characteristics in Optical Fiber Amplifier Using Fiber Mu-factor Controlxe2x80x9d in The Institute of Electronics and Information and Communication Engineers of Japan, Technical Report OCS94-66, OPE94-88 (1994-11) has been known as a conventional scheme.
The conventional scheme uses a fiber gain controller (AFGC) for monitoring the total output of four signal light beams subject to wavelength multiplexing and controlling the fiber gain such that the output level becomes constant. Through this, the inter-input wavelength difference is made to be 0 dB and the fiber gain is controlled to a constant value of 12 dB so as to minimize the difference between wavelengths. Further, by using an auto-power controller (APC) based on an optical attenuator 58, light loss is adjusted while keeping the fiber gain constant at 12 dB to make the fiber gain spectrum unchanged even when the relay mu-factor is changed.
Typically, in a practical system of wavelength multiplexing transmission, transmitting signal information pieces represented by wavelengths are often independent of each other. In this case, only necessary signals are transmitted and there is a possibility that unnecessary signals are stopped, that is, placed in standby condition.
However, in the conventional scheme in which the total output of multiplexed four signal light beams is monitored and controlled, when the number of multiplexed signal wavelengths is changed, the total output remains unchanged but outputs of signal light beams of individual wavelengths change greatly. With the outputs of signal light beams greatly changed in this manner, there arise problems that signal transmission is adversely affected and that when the outputs exceed a signal transmission distance limit, the signal transmission becomes impossible.
Further, in the conventional scheme in which the total light output is controlled, when the output of one signal light beam decreases, this decrease adversely affects the signal transmission.
An object of the present invention is to provide an optical amplifier which can control individual wavelength outputs without greatly affecting signal transmission even when the number of signal wavelengths subject to multiplexing changes and an optical transmission system using the optical amplifier.
Another object of the present invention is to provide a method of monitoring a probe light beam included in a multiplexed light ray propagated through a transmission path in the optical transmission system and controlling an output of the probe light beam and outputs of the multiplexed light beams from the optical amplifier in accordance with a result of monitoring.
Another object of the present invention is to provide an optical transmission path monitoring and controlling method which can decide whether or not an optical transmission path trunk line, a probe light beam per se, multiplexed light beams per se, an optical amplifier and a transmitter are normal by monitoring the probe light beam and other multiplexed light beams included in the multiplexed light ray propagated through a transmission path in an optical transmission system and detecting what values the probe light beam and the other multiplexed light beams take in relation to predetermined values, respectively.
To accomplish the above objects, the present invention comprises an optical amplifying medium to which input multiplexed light beams are guided, an exciting source for exciting the optical amplifying medium, light branching means for branching part of the multiplexed light beams amplified by the optical amplifying medium, light separating means for separating a probe light beam from multiplexed light beams branched by the light branching means, light receiving means for detecting an output of the probe light beam separated by the light separating means and converting the detected output into an electric signal, and control means for controlling an amount of operation of the exciting source on the optical medium such that an output of the light receiving means becomes constant, and with the construction as above, the output of the probe light beam can be rendered to be constant, the outputs of the signal light beams can be rendered to be constant and even when the number of signal wavelengths subject to multiplexing changes at that time, the individual wavelengths can be controlled without greatly affecting the signal transmission.
Preferably, the above optical amplifier further comprises second light receiving means for detecting outputs of multiplexed light beams branched by the light branching means, second light branching means arranged to precede the optical amplifying medium and operative to branch part of the multiplexed light beams guided to the optical amplifying medium, second light separating means for separating the probe light beam from the multiplexed light beams branched by the second branching means, third and fourth light receiving means for detecting the output of the probe light beam branched by the second light separating means and the outputs of the multiplexed light beams branched by the second branching means, respectively, and input detecting means for delivering a command to the control means when the output of the probe light beam detected by the third light receiving means is lower than a predetermined value and the outputs of the multiplexed light beams detected by the fourth light receiving means are higher than a predetermined value, and the control means controls the amount of operation of the exciting source on the optical medium such that the output of the second light receiving unit becomes constant, so that with the above construction, event when the probe light beam becomes abnormal, all of the multiplexed light beams can be controlled.
Preferably, the optical amplifier further comprises alarm means for informing abnormality, the input detecting means delivers a signal to the alarm means when the output of the probe light beam and the outputs of the multiplexed light beams are lower than the predetermined values, respectively, so as to actuate the alarm means, and with the above construction, even when abnormality occurs, the abnormality can be informed easily.
Preferably, in the optical amplifier, the optical amplifying medium is a rare earth added optical fiber, the exciting source is a pump laser, and with the above construction, the amplification degree for signal light beams can be increased.
Preferably, the optical amplifier further comprises correcting means for calculating a difference between outputs of the first and second light receiving units and correcting a reference value, included in the control means and used to control the probe light beam to a constant value, in such a manner that the difference becomes a predetermined value, and with the above construction, the output during laying can be made to be equal to that during initial adjustment.
Preferably, in the optical amplifier, the light separating means includes first and second optical couplers each having four ports and an optical filter for selectively passing a light beam of a predetermined wavelength, the multiplexed light beams branched by the light branching means are guided to the first port of the first optical coupler, multiplexed light beams delivered out of the fourth port are detected by the second light receiving means, the multiplexed light beams branched by the light branching means are guided to the first port of the first optical coupler, multiplexed light beams delivered out of the third port are guided to the optical filter and are further guided to the third port of the second optical coupler, the probe light beam delivered out of the second port is detected by the second light receiving unit, multiplexed light beams branched by the second light branching means are guided to the first port of the second optical coupler, multiplexed light beams delivered out of the fourth port are detected by the fourth light receiving unit, multiplexed light beams branched by the second light branching means are guided to the first port of the second optical coupler, multiplexed light beams delivered out of the third port are guided to the optical filter and further guided to the third port of the first optical coupler, and the probe light beam delivered out of the second port is detected by the first light receiving unit, and with the above construction, the construction of the light separator can be simplified.
Preferably, the optical amplifier further comprises combining means for combining the probe light beam delivered out of the separating means with light beams in the stage preceding the optical amplifying medium, and with the above construction, even when the probe light beam becomes abnormal, the succeeding trunk line system cannot be affected thereby.
The present invention comprises a light transmitting apparatus for multiplexing a plurality of signal light beams inclusive of a probe light beam and delivering a multiplex light ray to a trunk line system constructed of an optical fiber, an optical amplifier provided midway of the trunk line system and operative to separate the probe light beam from the light signal delivered out of the light transmitting apparatus and control the mu-factor for the input light beams such that an output of the probe light beam becomes constant, and a light receiving apparatus for receiving a light signal amplified by the optical amplifier and transmitted to the trunk line system, and with the above construction, even when the number of signal wavelengths to be multiplexed changes, signal transmission cannot be affected thereby and the individual wavelength outputs can be controlled.
Preferably, in an optical transmission system using the optical amplifier, the light transmitting apparatus includes a plurality of transmitters for transmitting signal light beams and a probe light beam transmitter for constantly transmitting the probe light beam independently of the plurality of transmitters, and with the above construction, outputs of the signal light beams can be controlled without affecting the signal light beams.
Preferably, in the optical transmission system using the optical amplifier, the light transmitting apparatus includes a plurality of transmitters for transmitting signal light beams, one of the signal light beams transmitted from the plurality of transmitters is used as the probe light beam, and with the above construction, the transmission amount of information of the signal light beams can be increased without requiring the provision of a separate probe light beam.
Preferably, in the optical transmission system using the optical amplifier, the outputs of individual wavelengths from the transmitters are made to be substantially equal to each other, and with the construction as above, the signal light beams can be controlled with ease.
Preferably, in the optical transmission system using the optical amplifier, the output of the probe light beam is made to be higher than the outputs of the signal light beams from the transmitters, and with the construction above, even when the transmission distance of the trunk lien system is increased and input signal light beams are decreased, the probe light beam can be detected to ensure that outputs of the signal light beams can be amplified and transmitted through the long distance.
Preferably, in the optical transmission system using the optical amplifier, the output of the probe light beam transmitter is made to be lower than the outputs of the signal light beam transmitters, and with the above construction, the outputs of the signal light beams can be increased.
Preferably, in the optical transmission system using the optical amplifier, a wavelength distance between the probe light beam and an adjacent signal light beam is made to be different from that between the signal light beams, and with the above construction, accuracies of the light outputs can be improved.
Preferably, in the optical transmission system using the optical amplifier, the probe light beam has a wavelength positioned at a short wavelength end or a long wavelength end of the signal light beams, and with the above construction, separation of the probe light beam can be facilitated.